Skeletal muscle hypertrophy induced by Low-Intensity Exercise with heat-stress in healthy human subjects

Abstract

Effects of low-intensity exercise (LIE) combined with or without heat-stress on muscle hypertrophy were investigated. Nine healthy men were subjected to 10 weeks of LIE for elbow flexor muscles{4 days/wk at less than 30 repetition maximum (RM), 3 sets (30 repetitions) of flexion-extension exercise of elbow joints}. The 60-min heat-stress was applied to only non-dominant arm, which performed LIE during the last 30-min period, by using a heat- and steam-generating sheet (heating area was set for 430 cm2, Kao Corporation, Tokyo, Japan). The dominant arm was subjected to only LIE. Maximum isometric force in flexion of the heated non-dominant, not the unheated dominant, arm significantly increased (18.4 %) after 10-week LIE (p < 0.05). However, the maximum isometric torque in extension of both arms was not improved. Mean cross-sectional area of biceps brachii muscles in the non-dominant, not the dominant, arm was significantly increased by LIE combined with heat-stress for 10 weeks (7.5 %, p < 0.05). It was strongly indicated that exercise training at an intensity even lower than 50 % 1 RM could be effective in increasing of muscle strength associated with hypertrophy, when the training was combined with heating. These results suggest that heat-stress might be a useful countermeasure for prevention of muscular atrophy during space flight and/or long-term bed rest. Its application could be also extended to rehabilitation.